The present invention relates generally to high pressure sodium lamps having jackets of sintered polycrystalline aluminum oxide. More particularly, it relates to modification of lamp structure and components to overcome a problem of loss of sodium pressure within the lamp envelope, and particularly the loss of sodium, and the reduction of the pressure of sodium vapor necessary to the favorable operation of the lamp.
As used herein the term deluxe, as it is used in reference to high pressure sodium or HPS lamps, means a lamp having a pressure of sodium substantially higher than that of standard or conventional HPS lamps. For convenience of reference DHPS is employed as an alternative designation to the phrase deluxe high pressure sodium as used in connection with lamp structures. The term also designates a lamp which emits a light which is substantially white as contrasted with the light emitted from standard HPS lamps. The light emitted from standard HPS lamps is characteristically golden in coloration. The term HPS is used in the conventional sense to indicate high pressure sodium lamps which operate at lower pressure than DHPS lamps and which emit a characteristic golden light.
Components for lamps for the generation of light, which may involve the use of sodium and particularly sodium in high pressure, are disclosed in U.S. Pat. Nos. 4,285,732; 3,026,177; 3,485,343; 3,026,210; 3,935,495; 4,079,167; 4,150,317 and 3,788,710. The text of these patents, which are assigned to a common assignee herewith, are incorporated herein by reference.
As is explained in the above patents, sintered polycrystalline aluminum oxide is used as the jacket materials for discharge tubes of lamps. Such lamps may contain high pressure sodium (HPS) or the higher pressure sodium of deluxe (or DHPS lamps) in the discharge tubes. It is possible to obtain the desired partial pressure of sodium in these tubes by using an amalgam of sodium in mercury.
One of the major factors limiting the life of lamps employing the high pressure sodium discharges is the loss of sodium from the discharge. When the partial pressure of sodium within the discharge tube of the lamp is reduced, the light output of the lamp is affected. When the loss of sodium from the vapor phase in the lamp is large, the lamp may not even light when electric voltage is applied to the lamp in the conventional manner to induce operation.
Further it has been observed that a lamp, which initially has a suitably high pressure of sodium for deluxe HPS use or for HPS use, may gradually lose its pressure over a period of lamp use. Thus, although a DHPS lamp operates well initially, the useful life of the lamp may be so limited as to make sale and use of the lamp in commerce uneconomical or impractical. The standard HPS lamps have a less pleasant golden color. The efficiency of operation of HPS lamps as well as the life expectancy of such lamps is affected by the maintenance of a suitable high pressure of sodium in the lamps.
To be a color improved HPS lamp, so called deluxe lamp (DHPS lamp, the lamp should operate with higher pressure of sodium and this pressure is two to three times the pressure of sodium in a standard or conventional HPS lamp. One advantage of such deluxe lamps is that they emit a light which is whiter than that emitted from the lower pressure standard HPS lamps.
Standard HPS lamps have lives of the order of 20,000 hours. Longer operating lives are, of course, desirable.
It has been observed that within 3,000 to 10,000 hours of operation of deluxe HPS lamps (DHPS lamps), they may lose their color advantage and revert to the standard HPS lamps which emit the unpleasant golden color.
A number of studies have been made and are reported in the literature which have been concerned with the mechanisms by which sodium is lost from high pressure sodium lamps. The following are a number of reports which have been made on this general subject:
(A) A. Inouye, T. Higashi, T. Ishigani, S. Nagamo, and H. Shimojima, Journal of Light and Vis. Env. 3 (1979) 1. PA0 (B) P. R. Prud'homme Van Reine, "Science of Ceramics", Proceedings of the Twelfth International Conference, June 27-30, 1983, Saint-Vincent, Italy, P. Vincenzini (Ed.), Ceramurgica, Italy, 1984, p. 741. PA0 (C) E. F. Wyner, Journal of IES, 8 (1979) 166. PA0 (D) H. Akutsu, Ph.D. dissertation, "Development of High Pressure Sodium Lamps", Matsushita Electronics Corp., Osaka, Japan, 1982. PA0 (E) F. C. Lin and W. J. Knochel, Journal of IES, 3 (1974) 303. PA0 (F) P. Hing, J. Illum. Eng. Soc. 10 (1981) 194.
In the first article, identified as A above, the suggested mechanism for the reduction in the pressure of sodium vapor is one by which leakage occurs through the seal glass. According to references C and E involved, the suggested mechanism for the loss of the sodium of the high pressure sodium vapor is by electrolysis through the tube wall.
The mechanism suggested in the references of D and F is one according to which a reaction occurs with the tube wall and diffusion occurs through the wall. Many investigators believe that the sodium loss occurs by this latter mechanism.
These latter references also suggest that sodium present in the arc tubes reacts with the alumina of the enclosing tube to form beta alumina having formula Na.sub.2 O.11 Al.sub.2 O.sub.3 and/or sodium aluminate having the formula NaAlO.sub.2.